1. Field of the Invention
This invention relates generally to a synchronization system for a wireless transmitter more particularly, the invention relates to a system for synchronizing a portable transceiver to a network in both frequency and time with limited crystal tuning.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset, also referred to as a portable transceiver. The different modulation and transmission schemes each have advantages and disadvantages. These portable transceivers each communicate with a communication network via a base-station, located in what is typically referred to as a “communication cell” or “cell.”
As mobile communication systems have been developed and deployed, many different standards have evolved, to which these systems must conform. For example, in the United States, many portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme is narrow band offset π/4 differential quadrature phase shift-keying (π/4-DQPSK), and the access format is TDMA.
In Europe and increasingly in other countries, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift-keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology. Other modulation formats use minimum shift keying (MSK), frequency shift-keying (FSK) and other shift-keying modulation methodologies.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient, non-linear power amplifier can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying amplitude) modulation signal.
Regardless of the type of communication methodology used, each portable transceiver must maintain frequency and timing synchronization with the communication network to which it is connected. For example, in GSM, the portable transceiver must maintain a carrier frequency accurate to within 0.1 part per million (ppm) of that of the base station. Also, the portable transceiver must maintain its time base to within 2 microseconds (μs) of the time base of the base station. The time base of the portable transceiver is adjusted in increments of ¼ of a symbol period (0.9225 μs) in intervals of Δt such that 1 s<Δt<2 s until any error is less than ½ of a symbol period. Many portable communication handsets maintain synchronization to the network by using a tunable crystal in a closed tracking loop. For example, some systems use a costly temperature controlled voltage controlled crystal oscillator (TCVCXO), and some use a varactor to adjust a voltage controlled crystal oscillator (VCXO). A varactor controlled oscillator is less accurate than a capacitor controlled oscillator and usually requires a more costly digital-to-analog converter (DAC). A capacitor controlled oscillator is adjusted using a set of capacitor arrays, which effectively form a DAC. In such a system, the crystal is adjusted, typically using a complex array of fixed and adjustable capacitances providing coarse and fine tuning capability using feedback from the network. For example, in the GSM communication network, some portable communication devices use a capacitor controlled crystal oscillator (CCXO) to maintain frequency and timing synchronization with the network. Unfortunately, highly accurate crystals are costly, and complex capacitor arrays for tuning the CCXO consume valuable area on the integrated circuit device on which they are located.
Therefore it would be desirable to reduce the required accuracy of the crystal oscillator and to reduce the complexity of the tuning circuitry used to adjust the crystal oscillator and still maintain frequency and timing synchronization with a communication network.